RU2646180C1 - Method for thermocyclic treatment of steels - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: at least two heating cycles are performed for quenching to austenite homogenization temperature and cooling at rate providing martensitic transformation and tempering with heating at a rate higher than 50°C/sec up to a temperature not higher than Ac₁. At the first cycle, heating is carried out up to austenization temperature with holding till complete homogenization of austenite, at the second and subsequent cycles, high-speed heating is carried out under hardening at the rate of 50°C/sec without soaking to the temperature, providing austenite homogenization, the tempering temperature in each subsequent cycle is lower than in the previous one. The last tempering is carried out at a temperature ensuring the required properties of steel.

EFFECT: extended service life of machine parts and tools made of alloyed, low-alloyed and carbon steels.

1 ex

Description

The invention relates to metallurgy and mechanical engineering and can be used for heat treatment of steel products. The objective of the invention is to increase the service life of machine parts and tools made of alloyed, low alloy and carbon steels.

A known method of the METHOD FOR THERMOCYCLIC TREATMENT OF INSTRUMENTAL STEEL (p. 2131469, RU), which consists in repeatedly heating the samples above A _C1 at 130-170 ° C at a speed of 6-35 deg / s, cooling in a cycle below A _{C1 is} carried out in a molten salt to a temperature 680-750 ° C with holding at this temperature for 3-9 minutes, cooling in oil after heating in the last cycle with subsequent tempering is carried out at a temperature of 200-400 ° C. The disadvantages of this method are the need to use a toxic molten salt and the inability to obtain submicron and nanometric structures.

A known method of high-temperature thermocyclic processing (cyclic electrothermal treatment), which consists in electric heating at a speed of about 50 K / s to a temperature of complete austenization, cooling at a speed of 30-50 K / s to a temperature of 420-450 ° C, corresponding to the temperature of the fastest isothermal decomposition of austenite and holding it at this temperature. At the end of the exposure, the cycles are repeated, in the last thermal cycle they are quenched from the austenitic state (Fedyukin V.K., Smagorinsky M.E. Thermocyclic treatment of metals and machine parts. - L .: Mashinostroyenie, 1989, p. 27). The disadvantages of this method: to obtain a particularly finely dispersed steel structure requires repeated repetition of cycles, because with isothermal decomposition of austenite, pearlite grains are obtained, which are hereditarily large (grinding occurs only due to phase hardening); it is impossible to obtain a submicron - nanocrystalline structure, because temporary exposure is used in thermal cycles, leading to grain growth.

The closest solution is the method of thermocyclic processing of steel products (No. 2594925), characterized in that it includes at least two heating cycles for quenching at a rate above 50 ° C / s to the temperature of austenite homogenization without holding, cooling at a speed that ensures martensitic transformation and tempering with heating at a rate above 50 ° C / s to a temperature below A _c1 to obtain a finely divided structure of the steel product, and in each subsequent cycle, quenching is carried out to a temperature lower than in the previous m

The disadvantage of this method is pulsed heating without holding in the first cycle. As a rule, pulsed heating is designed to form surface layers. For products whose thickness exceeds the heat affected zone, it is often necessary to heat-treat the internal volumes to give the required strength to the entire product.

The essence of the invention consists in performing at least two cycles of quenching to the temperature of homogenization of austenite and cooling at a rate that ensures martensitic transformation, and tempering with heating at a speed above 50 ° C / s to a temperature not higher than Ac1, according to the invention, in the first heating cycle it is carried out up to the austenitization temperature with holding until the austenite is completely homogenized; in the second and subsequent cycles, high-speed quenching is carried out at a speed of 50 ° C / s without holding to the temperature, providing homogenization of austenite, the tempering temperature in each subsequent cycle is lower than the previous one.

First cycle:

- heating to a temperature above A _s3 , but below the solidus temperature (steel melting);

- temporary exposure at this temperature until complete homogenization of austenite in the entire volume of the product;

- cooling in an environment providing martensitic transformation;

- short-term heating of steel to a temperature below A _c1 , ensuring the decay of martensite (first vacation);

Second cycle:

- high-speed pulsed heating of steel with a speed of at least 50 K / s to the temperature of complete homogenization of austenite (significantly higher than A _c3 ), but lower than the solidus temperature (steel melting), and the heating temperature is higher, the higher the heating rate;

- after the end of heating - cooling in the medium or cooling due to heat removal to the inner layers of the metal, providing martensitic transformation;

- short-term heating of steel to a temperature below the heating temperature during the first tempering in the first cycle, which ensures the decomposition of martensite (second tempering).

The next cycle is similar to the previous one. The difference is a decrease in the temperature of the previous impulse heating for quenching. This is due to an increase in the dispersion of the steel structure after the previous cycle, which requires less time for the homogenization of austenite. The decrease in tempering temperature in the second and subsequent cycles is also associated with an increase in the dispersion of the structure and is aimed at obtaining high hardness and dispersion of steel. For example, in the first cycle, the maximum tempering temperature of steel corresponds to the temperature of high tempering, in the second - average tempering, and in the third - low tempering.

Thus, in the first cycle after quenching, a coarse-grained structure is obtained, as a rule, rack or needle martensite, which after high tempering breaks up to form a troostor sorbitol structure.

When performing the last cycle, the last operation is tempering, which provides the specified structure and properties of steel.

The number of cycles is determined by the required final properties of the steel.

The essence of the processes occurring under pulsed heating is that when heated to high temperatures, austenite is homogenized, and a high heating and cooling rate prevents the growth of austenitic grain. Short-term heating for tempering ensures the formation of martensitic grains in place of a troostite or sorbitol structure, the grain size of which is smaller than the initial ones. The grinding of the initial structure is due to the formation of many new grain growth centers in place of the initial grain, the growth of which ceases upon cooling without aging, which leads to a significant refinement of the structure. Short heating during tempering reduces internal stresses, leads to the decomposition of residual austenite and inhibits the growth of pearlite grains.

The use of pulsed heating significantly reduces the heat treatment time. When performing the operations described above, the dispersion of the steel structure significantly increases up to a submicron value. After the last vacation, as a rule, “structureless” martensite is formed.

Example

First cycle:

A shaft with a diameter of 50 mm from st45 is heated in a furnace to a temperature of 850 ° C, and is aged for 1 hour. Quenching is done in water. Vacation temperature - 400 ° С, vacation time - 1 hour. Air cooling.

Second cycle:

The product is heated to a temperature of about 1100 ° C at a speed of 70 ° C / s and immediately after heating without quenching it is quenched in water. Next, a vacation is made - heating of the high-frequency substance to a temperature of 400-420 ° C at a speed of 50-70 ° C / s and cooling at a speed that prevents the growth of pearlite grains (in water or in air).

If the first two cycles do not provide the required properties of the steel, then an additional one or more cycles are performed according to the same principle, followed by a decrease in the temperature of quenching and tempering.

In the last cycle, the temperature and duration of tempering are selected from the conditions of the desired final structure and properties of the steel. As a rule, an unstructured tempering martensite is obtained (the grain structure is not detected using optical microscopes - the grain size is less than a micron), trostan-martensite or a particularly finely dispersed troostite structure depending on temperature and time exposure.

Claims (1)

A method for thermocyclic processing of steel products, including hardening cycles until the austenite is homogenized and cooling at a rate that ensures martensitic transformation, and tempering with heating at a speed above 50 ° C / s to a temperature not higher than Ac ₁ , characterized in that in the first cycle heating to austenitization temperature with exposure to complete homogenization of austenite; in the second and subsequent cycles, quenching is carried out at a rate of at least 50 ° C / s without the above exposure, and heating under tempering in each subsequent the cycle is carried out to a temperature below the previous cycle.